Suppose you are going to toss a coin three times.
This could be thought of as a series of three events.
 
 
 
The tree diagram to the left shows all the possible outcomes of the 3 tosses of the coin.
 
The branches also show the chance of going down that branch (getting that particular outcome).
 
So the chance of getting HHT is 0.5*0.5*0.5 or 0.125